1. Field of the Invention
The present invention relates to an x-ray tube for use in particular in the medical field. The main characteristics of these tubes are resistance to drift of their emission characteristics as a function of their temperature as well as homogeneity of the x-ray illumination produced by all the points of their focus. The aim of the invention is to improve such tubes while guarding against any danger of destruction under the action of overheating of their anode or of their cathode.
2. Description of the Prior Art
In general terms, x-rays are produced by electron bombardment, within a vacuum enclosure, of a target fabricated from material having a high atomic number. The electrons which are necessary for bombardment of said target are liberated by thermoelectronic effect, usually in a helical filament of tungsten, of a cathode placed with precision within a concentration component. The concentration component performs a focusing function at the same time as a Wehnelt function. The target constitutes an anode of the x-ray tube. In this very conventional type of configuration, the initial velocities of the electrons at the level of the emitter are highly dispersed. The electron trajectory therefore has a disordered structure and the focusing system provides a correcting function but does not usually achieve sufficiently high performance characteristics. In consequence, instead of an impact of bombardment electrons on the target, there is obtained a fairly complicated entanglement of trajectories. This provides the thermal focus of the x-rays with an energy profile which is hardly compatible with good quality of the image.
In recent developments, for example in those described in European patent Application No. 85 106753.8 filed on May 31 1985, reference is made to a cathode which is no longer constituted by a filament but is now constituted by a portion of strip provided for emission of electrons with a flat surface located opposite to the anode. The advantage of employing a flat electron emitter has already been presented prior to this Application. It consists in maintaining a certain cohesion of the electronic charges during their trajectory towards the target. Experience has in fact shown that there is obtained in this case a distribution of electrostatic potential which is conducive to better focusing of the electric charges. The x-ray focus thus obtained accordingly exhibits a practically homogeneous energy profile which has a favorable effect on the quality of the image. The scientific literature records certain experiments which are based on this general principle and in which use is always made of an emitter constructed in the form of a tungsten strip. However, these strips are systematically attended by problems of thermomechanical strength. It was in fact with a view to solving such problems that the European patent Application cited above was filed. In particular, in spite of all the care and attention devoted to rolling of the strips, these strips are subjected to differential stress phenomena and, as a result of successive heating and cooling within the x-ray tube, acquire a so-called corrugated-sheet appearance. The advantages arising from the use of a flat emitter are then lost.
The object of the present invention is to overcome this disadvantage by proposing a flat emitter device which offers high mechanical strength and thus makes it possible to remove the corrugated-sheet problems mentioned earlier. To simplify, the emitter is constituted by a beam. This beam is preferably of hollow construction and may have a substantially rectangular cross-section. It is thus possible to benefit by all the advantages offered by the rigidity of a beam, such rigidity being substantially greater than that of a strip. Furthermore, in order to avoid the need to heat an excessively large mass of material, the beam is of hollow construction. In respect of a given heating power, this reduces the turn-on time of the x-ray tube. In one improvement, the hollow beam is even traversed by a helical heating coil from one side to the other and the beam is thus heated by indirect heating. This indirect heating can even be focused only on predetermined portions of the beam, especially the beam face located opposite to the anode. This permits a further limitation of the heating power.